Shipping container explosives and contraband detection system using nuclear quadrupole resonance

ABSTRACT

For container-carried explosives or contraband, stimulated emissions due to nuclear quadropole resonance are detected utilizing a terminated balanced transmission line and a directional coupler for the detection of explosives, contraband, narcotics and the like that exist in metal containers.

RELATED APPLICATIONS

This application claims rights under 35 USC §119(e) from U.S.Application Ser. No. 61/299,673 filed Jan. 29, 2010, the contents ofwhich are incorporated herein by reference.

FIELD OF THE INVENTION

This invention relates to detection of explosives or contraband in ashipping container, and more particularly to the use of nuclearquadrupole resonance for the detection of molecules such as explosives,narcotics and other molecules of interest contained in metallic shippingcontainers.

BACKGROUND OF THE INVENTION

The use of shipping containers to house explosives or contrabandpresents certain detection problems, especially when the containershouse non-nuclear explosives and contraband such as drugs and the like.Priorly detection of explosives or contraband in a shipping containerrequired opening the container for inspection. This however is acumbersome process and there is a requirement for remote substancedetection that can penetrate a metal container. While various techniquesmay be available, nuclear quadrupole resonance has been found to workwhen using terminated balanced transmission lines such as described inco-pending patent application entitled Transmission Line Array ForExplosive Detection Using Nuclear Quadrupole. Resonance, BAEP-1288,filed on even date herewith and incorporated herein by reference.However, the ability to use the technique to penetrate metal shippingcontainers was thought impossible.

By way of background, in the early 1900s, not long after Einsteinpublished his equations on thermal equilibrium, individuals realizedthat there were likely to be resonances at very low frequencies foratoms and molecules and that these resonances would occur because if oneemits a photon of exactly the correct frequency, the material willabsorb this photon, store it for some amount of time and then get rid ofthe absorbed energy. It is has been found that in nature the moleculeswhich absorb such energy always fall to a lower energy state.

One of the ways for the material to emit energy is through spontaneousemission where a photon of exactly the same energy that is impinging onthe material is thrown off in a random direction at random times.

The second way of getting rid of the energy absorbed by the material isthrough process of stimulated emission in which a photon arrives atexactly the appropriate energy, gets near the molecule, stimulates themolecule and when the molecule drops to the lower energy state it emitsa photon that is exactly in phase with the original photon.

The energy that is thrown off either in spontaneous emission orstimulated emission results in an exceedingly narrow spectral line. Infact the line is generally considered to be a single line that exists ata given wavelength or frequency. It is noted that the material only hasone choice assuming that the material is pumped at its lowest energystate, raising the energy within the molecule such that the only waythat it can release its energy is to emit a photon of that exact energy.

Nuclear quadrupole resonance has been utilized in the past to detect thepresence of specific molecules, including explosives. Explosivesgenerally involve the use of nitrogen or nitrogen bonded with otherelements. When nuclear quadrupole resonance was utilized in the past, itwas used to detect the presence of molecules due to the molecularelements that are bonded together such that the molecules absorb energyat for instance as many as eight different energy levels or spectrallines. It turns out that at least three of the energy levels tend to beprominent, although in some materials there are upwards of all eightenergy levels for one bond. If one has many bonds there may be manydozens of spectral lines. In order to detect the presence of a moleculeone usually is looking to pump energy right at the top of one of thespectral lines and look for energy coming back at the same frequency.

As discussed in co-pending patent application by Paul A. Zank and JohnT. Apostolos, entitled Method and Apparatus For Sensing The Presence OfExplosives, Contraband and Other Molecules Using Nuclear QuadrupoleResonance, BAEP-1277, filed on even date herewith and incorporatedherein by reference, as part of the subject invention, it has been foundthat the spectral lines of interest especially for explosives are in the100 KHz to 10 MHz range. A particularly interesting explosive is calledRDX which has a spectral line in the 3 to 4 MHz range, as does sodiumnitrate.

However if one is seeking to detect stimulated or emission orspontaneous emission at 3 MHz, the wavelength of the returns isincredibly long, in some cases corresponding to the size of a building.Moreover, the photons that are emitted in either spontaneous orstimulated emission represent very little energy. For instance, a redphoton carries an energy of about 3.5 electron volts, with detectableradiation being one or two millionths of 3.5 electron volts. The resultis that photons emitted from the molecules are virtually undetectable.One of the reasons is that in order to detect single photons one isfaced with thermal background that overwhelms the detection process. Inorder to achieve any type of result, one pumps large numbers of photonsinto the target material such that for every milliwatt second anextraordinary number of photons are involved.

If the photons are at the appropriate frequency they are absorbed andonly when the frequency exactly corresponds to a resonance line does themolecule start absorbing the photons. Thus it is quite important thatthe frequency source utilized in the nuclear quadrupole resonancemeasurements be extremely precise and stable.

If one performs a frequency sweep, the emission that comes back is onthe order of 1% of the energy that impinges on the molecule.

It is noted that prior nuclear quadrupole resonance techniques can belikened to looking into a headlight to find a 1% response.

As a result, a pulsed coil prior art nuclear quadrupole resonancedetection of molecules requires upwards of 100 kilowatts of energycoupled to a very high Q tuned coil having for instance a Q of 80 orbetter. If there is any offset in terms of the frequency of the incidentradiation or if the coil tuning was not precise, then any emissions fromthe molecule will be lost in the clutter.

First and foremost in the prior art pulsed coil nuclear quadrupoleresonance techniques, it was only with difficulty that one could in factdetect any response. One of the reasons is because the coil exhibits alarge dwell time after which one looked for a response.

If one did not wait, the incoming radiation would swamp the detectableresults. In order to eliminate this problem, those in the past used apulsed source and then waited for a response after the trailing edge ofthe pulse. Prior systems thus pumped pulsed energy into a coil with thetarget material at the center of the coil. Thereafter the material wouldabsorb energy and then the prior systems would listen for thespontaneous decay.

The problem with spontaneous decay that at thermal equilibrium aspontaneous photon happens only once for every two million stimulatedphotons. Thus, in terms of detecting spontaneous decay, one is at anextremely difficult power disadvantage. Secondly, the spontaneous decaymight happen over several tens of milliseconds which means that theinstantaneous power levels at any point in time are very low. Forspontaneous decay using a pulsed coil nuclear quadrupole resonance, theproblem is that one is working with very few photons and, further theyare stretched out over time. This means that one has to use huge amountsof power to overcome these problems, often in the nature of kilowatts ofenergy. Moreover, because one is looking at very low signal strength thecoil is made with a very high Q. This means that the coil couples wellwith the environment, that in turn means that the coil picks up a greatdeal of background noise.

Pulsed coil nuclear quadrupole resonance detection systems have beenmarginally cost effective and their power density has exceeded humansafe limits.

More specifically, taking RDX as an example, the bandwidth of the RDXresonance is about 400 hertz. This means that the associated decay timeor relaxation time is on the order of 2.5 milliseconds. If one were tosweep the frequency through the resonance as one approaches the resonantfrequency, what happens is that one excites the nucleus of the nitrogenatom. When the nucleuses are excited they go into an upper state andthen as one sweeps by the frequency there is a population inversion inthese nuclei at which time they start to decay.

If one utilizes a long CW pulse what would happen is that one would seea periodicity of absorption and emission. When the CW pulse is turnedon, the molecule goes into the excited state but then relaxes throughstimulated emission. What would happen utilizing a CW signal is that onewould see a series of absorptions and emissions that would occur every2.5 milliseconds.

For RDX, assuming a pulsed coil system, one must use a pulse width ofabout half a millisecond because the pulse has to decay down fast enoughso that the spontaneous emission can be observed.

Thus in the past a relatively short pulse of CW energy was used toenable listening for the response. However, in order to be able todetect the response at all, a very high Q coil was required. High Qcoils have an excessive relaxation time. As a result, in order toprovide for the ability to listen when driving a very high Q coil athalf a millisecond one has to have other circuitry to quench the coil asfast as possible so as to be able to listen to the return, typically interms of a little hiss that comes off after irradiation with the pulse.

Thus, in the prior systems one had to have exceedingly large kilowattsources of 3 MHz energy in order to obtain enough of a response, andthen had to pulse the source so as to be able to stop it and quench itin time to be able to detect the minuscule response that would occur.

Having the high Q coil further was complicated by the fact that onecould not frequency sweep a sample because the high Q coil resonates atonly one frequency.

This for instance precludes the ability to distinguish between thedetection of multiple spectral lines to be able to distinguish thespectral response of the target molecules from the spectral responsesfrom uninteresting molecules.

Also, when using a high Q coil one has to use an exceedingly largeamount of shielding to make the system safe for use around people, aswell as having to actively quench the coil.

Moreover, when pumping 1 kilowatt into a coil, the presence of thesystem is very easy to detect. Thus, terrorists could avoid screeningknowing that such a detection system was in operation.

Note that the pulsed coil system detects spontaneous not stimulatedemissions. Spontaneous emissions are not coherent and one obtains thesquare root of the power coming back.

Thus, in the past it has been virtually impossible to provide a workablesystem that would reliably and safely detect dangerous amounts ofexplosive material hidden on a human.

Substance Detection Using Nuclear Quadrupole Resonance and TerminatedBalanced Transmission Lines

As described in a co-pending patent, application by Paul A. Zank andJohn T. Apostolos, entitled Method and Apparatus for Sensing thePresence of Explosives, Contraband and other Molecules Using NuclearQuadrupole Resonance, filed on even, date herewith and incorporatedherein by reference, this application being based on ProvisionalApplication Ser. No. 61/299,652, an array can be provided for nuclearquadrupole resonance detecting systems in which an array of loaded orterminated balanced transmission lines is used for wide area coverage.In one embodiment, side by side balanced transmission lines aresimultaneously driven in phase with synchronous frequency swept signals.Each of the balanced transmission lines is fed with a low power sweptfrequency source and stimulated emissions are picked, off with adirectional coupler. For location, if a crossed grid array is used, thelocation of the sensed substance at a cross point as well as itsexistence can be sensed over a wide area. Alternatively, if a phasedetector is used for each balanced line, the phase between outgoing andincoming signals translates to the location of the sensed substance,measured from the feedpoint of the balanced transmission line.

Note, rather than using the high power noise-prone pulsed coil systemfor detecting nuclear quadrupole resonance lines due to spontaneousemission, in the subject system stimulated emission is sensed. Forstimulated emissions, the energy produced by the molecule is notspontaneous and it is not happening randomly. Rather, the emission thatis seen in the stimulated emission is coming back exactly in phase withthe incident radiation, namely a coherent response.

More specifically, a low power swept frequency source is used incombination with a probe in the form of a terminated balancedtransmission line in which molecules including explosives, narcotics andthe like that are located between the transmission line elements aredetected. In this system the result of the absorption of themilliwatt/watt energy is picked off with a directional coupler orcirculator so as to eliminate the transmitted energy from swamping thereceived energy. What is seen is the 1% stimulated emission coherentresult that is exactly in-phase with the transmitted signal. It is thecoherent in-phase relationship that permits integrating the weak signalsinto a detectable result.

As a result of utilizing the directional coupler the transmitted signalis rejected. Moreover, the utilization of a balanced transmission lineessentially has a zero Q, thus eliminating the background noiseassociated with the high Q coils. Moreover, since the transmission lineis not resonant at any one frequency, a sample can be frequency swept orsimultaneously irradiated with signals at multiple frequencies.Additionally, there is no frequency limit to the sweep frequency sincethere are no tuned circuits involved.

In one embodiment, the energy is step wise swept so as to be able tocorrelate the result with spectral lines of a known molecule while beingable to reject returns from molecules having other spectral lines.

It has been found for explosives such as TNT, RDX and PETN and othermolecules of interest that sweeping between 100 KHz and 10 MHz is enoughof a sweep to discriminate against non-target materials. For instance,while one might be looking for the spectral lines associated with RDX,one would also like to be able to ignore the spectral lines of for othermaterials, or for that matter glycine which is present in a great manybiologic materials.

This system is typically operated at between 200 milliwatts up to 10watts, making the system much safer than the high power kilowatt pulsedcoil nuclear quadrupole resonance systems. Moreover, quenching isunnecessary.

For robust detection of the stimulated emission, more than one spectralline can be considered as an indicator of the molecule. For instance,for RDX one might wish to look at two or three of the RDX spectrallines. If it turns out that glycine is present, and if in fact one ofthe RDX spectral lines share a spectral line with the glycine, then onecould ignore the overlapping spectral line.

While scanning network analyzers can be utilized as frequency sourcesfor the subject invention, due to the fact that the transmission linedoes not discriminate from one frequency to the next, it is possible toconnect multiple frequency sources in parallel to feed the transmissionline resulting in simultaneous evaluation of several frequencies.

It is also possible to use a pseudo-random number code pattern so thatthe system would be difficult to jam. Moreover, the low power system ishard to detect, obscuring the fact that any scanning is going on at all.

In one embodiment while one could scan from 100 KHz to 10 MHz, this typeof scanning procedure wastes a large amount of time and is notnecessarily beneficial. If one is only looking for specific resonancelines, the scanning can be scheduled to appropriately frequency hop,thus dramatically reducing scanning time.

Note in this system that no single detection of a spectral line is usedto declare the presence of the target material. Rather, the systemdesirably requires multiple hits in order to declare the presence of thetarget material.

It is also noted that this system looks at the stimulated emissions, asopposed to the spontaneous emissions, primarily because the spontaneousemissions are perhaps one two millionth of the power of the stimulatedemissions. This is important because, as mentioned above, in determiningthe presence of a target molecule, one is seeing only 1% of the incidentenergy being returned.

Further, RDX resonances have a bandwidth of approximately 400 hertz,which as mentioned above, results in a decay time of relaxation time ofabout 2.5 milliseconds. Assuming a stepped sweep approach, the nucleusof the atoms making up the molecules are excited and when they go intothe upper state, there is a population inversion in these nuclei, withthe stimulated emission occurring immediately thereafter. Note that thestimulated atoms that have been inverted relax coherently such thatthere is a coherent response back to the probe. Because of the 2.5millisecond relation time stepped sweeps would have to be adjustedaccordingly.

Since there is no coil involved, one does not have to use quenching andsince one uses a directional coupler to ignore the transmitted-signal,one does not have to stop and listen in order to get adequate readings.

Moreover, in one embodiment of this invention, a cancellation algorithmis utilized in which the transmission line is observed without a samplebetween the transmission line elements during a calibration sweep.Thereafter, any material that is between the transmission line elementshas results that are subtracted from the calibration sweep results.Thus, if there are any peculiarities in the analyzer or transmissionlines, these peculiarities are subtracted out. As a result, steady statenoise is nulled out.

The reason for the use of the transmission line is that it focuses allthe energy between the two balanced leads or elements. Because abalanced transmission line is the world's worst antenna by design itdoes not leak energy to the environment, unlike a coil. Concomitantly,the transmission line does not receive interference from theenvironment, making the subject system an extremely quiet system.

The system is implementable in a number of different forms such asproviding two spaced apart transmission line elements to either side ofa gate or portal through which an individual is to pass. Such a portalmay be an airport security checkpoint. Moreover, two pieces of copperpipe or copper tape may be placed on opposing walls down a corridor toform the transmission line, or the balanced transmission lines can beplaced on a road to detect the passage of target material between thetransmission line elements. Additionally, the transmission line couldfor instance be configured as opposed guard rails.

Considering for instance that a terminated balanced line contains twoelements, one element is called a plus element and the other is called aminus element. The magnetic flux lines are in a plane perpendicular tothe axis of the elements. In one configuration, a large area can becovered using a number of side-by-side plus/minus lines. For instance,these lines could be laid out in a carpet at an airport to track peoplecarrying explosives on their person. Thus, one can monitor thetransmission lines to be able to tell where someone carrying explosivesis walking and to be able to track their path.

It will be appreciated that this system, by avoiding the high Q coil,also avoids the large amount of shielding necessary for public safety orthe safety of those operating the equipment. Also, as mentioned above,there is no need to actively quench any part of the probe in order to beable to listen to the relatively small, returns from the irradiatedsample.

Rather than having to run a kilowatt into a coil, in the subjectinvention successes have been reported at a 200 milliwatt level withexcellent signal to noise ratios. Thus, there is the ability to operateat a 30 dB lower power levels than a pulsed coil. This means that theentire system can be run at low power. The result is that the subjectsystem does not interfere with magnetic media or people's safety and isvery hard to detect any distance away from the test site. Thus, evenstanding a few feet beside the balanced transmission line one is notable to detect it. As a result, a person would not know that he or sheis being monitored. Also, if a pseudo-random hopping schedule isutilized, detection of the presence of such a system is virtuallyimpossible.

The amount of power required is dependant on how much material one istrying to detect and also the flux density that one is trying to exciteit with, as well as how much integration time is available.

Small amounts of explosives can be carried on the person in the personsclothing, swallowed, or can even be surgically implanted, which would bevirtually undetectable through a physical examination of the person andalso through standard X-ray techniques. Thus for the creative ordiligent terrorist, it may be of interest to provide pockets of theexplosive within the body of the individual that could not be readilydetected by present techniques.

It is noted that the maximum flux density given two spaced apartconductors is on a line between the two conductors, with the minimumbeing outside the transmission line. As one proceeds to the edge of theconductors, one obtains more flux density. However, the flux densitydoes not very significantly in a direction normal to the plane betweenthe two transmission line elements so it is possible to get reasonablecoverage for a human sized object or even a truck sized object above thetransmission line. Note that the transmission line impedance cantypically be between 100 and 1,000 ohms which is not critical. Thecritical component is the flux density, with the critical flux densitybeing, approximately 1 watt per meter².

With a flux density of less than 1 watt per meter², the signal-to-noiseratio is less for the same integration time. If the flux density isgreater than 1 watt per meter², then the signal-to-noise ratio isimproved because of the coherent signal. The result of the coherency isthat the signal-to-noise ratio improves linearly with how muchintegration time is utilized.

Integration time refers to the collection of the results of multiplestimulated emissions over time. As a general rule, one has to dwell onthe target material for whatever is the inverse of the particularbandwidth involved. Bandwidths in the subject case are on the order of a100 to 500 hertz which results in dwell times of between 1 and 5milliseconds.

Of course, as mentioned above, one need not frequency hop in 1 to 5millisecond intervals because there is no reason why one cannot monitormultiple lines simultaneously or even feed the lines withparallel-outputted frequency generators. In short if one were usingthree signal generators coupled to the same transmission line, one couldsense three different spectral lines simultaneously.

Since this system can sample multiple frequencies simultaneously this isconsiderably different from the pulsed coil nuclear quadrupole resonancesystems that tend to tune a coil for a specific frequency because of theneed for the high Q. Thus, in the subject system one can track theresults over the entire bandwidth utilizing the same balancedtransmission line probe.

As a result, this system is capable of detecting an entire class ofexplosives, whether they are people-born or vehicle-born. Moreover, thesubject system may detect contraband such as narcotics, with manynarcotics having very specific nuclear quadrupole resonance signatures.This includes cocaine and heroin.

It will be appreciated that for some complex organics the spectral linestend to be larger, such as those associated with glycine. Glycine, evenin its usual 5% concentration for dietary supplements, for instance, canbe distinguished by recognizing the glycine spectra and subtracting outthe nuclear quadrupole resonance signature. As a result, if it turns outthat one of the spectral lines happens to be right on top of themolecule of interest, the subject system provides way to discriminateagainst the non-target molecules.

In summary, stimulated emissions due to nuclear quadrupole resonance aredetectable utilizing an array of terminated balanced transmission linesand directional couplers, thus to detect explosives, contraband,narcotics and the like that exist between the transmission lineelements, as well as to locate detected substances. In one embodiment, astepped frequency generator is utilized to provide a scan between 100KHz and 10 MHz. In another embodiment, parallel frequency sources arein-phase coupled to the balanced transmission lines, either embodimentpermitting correlation with expected spectral lines, with the frequencysources being low power so as to not create a safety hazard and so asnot to interfere with radiation sensitive devices such as film orelectronic circuits that are in the vicinity of the balancedtransmission lines.

What is not clear from the above is whether such a system would be ableto detect substances in a metal-walled shipping container.

SUMMARY OF THE INVENTION

While the above discusses nuclear quadrupole resonance in detectingcontraband or explosives utilizing for instance a grid array ofterminated balanced transmission lines, it is a finding of the subjectinvention that contraband or explosives within a metal shippingcontainer can nonetheless be detected utilizing these, techniques, dueto the very deep skin depth of metal containers at 0.1-4 MHz whichresults in increased penetration of the 0.1-4 MHz signals within thecontainer.

The result is that the container is relatively transparent to signals at0.1-4 MHz which permits the detection of substances utilizing nuclearquadrupole resonance.

One of the reasons that contraband and explosives can be detected in ashipping container is that shipping containers usually contain a largeamount of the substance to be detected. Thus, large amounts ofexplosives, for instance greater than 500 pounds, can be detected by thesubject system.

Moreover, the system is effective in detecting contraband and explosivesbecause of the multi-pass reinforcement within the shipping container ofthe 0.1-4 MHz signal that penetrates into the container. It has beenfound that the multi-pass reflections intensify the stimulated emissionsof the material within the container. This increased stimulated emissionresults in a signal that leaks out through the metal container and isdetectable by the subject nuclear quadrupole resonance system.

Thirdly, since the shipping containers are in transit for significantperiods of time, long integration intervals are available to permitrobust container-carried substance sensing.

Thus, while it might be thought that metalized shipping containers wouldpreclude the detection of substances utilizing NQR and stimulatedemission, it has been found that due to the deep skin depth andresultant penetration associated with the metal containers, thecontainers are essentially transparent in the 0.1-4 MHz range. Thismakes getting the signal into the container and getting the stimulatedemission signals out of the container easy. Moreover, any attenuationthrough the skin of the shipping container is counteracted by themultiple pass reinforcement of the 0.1-4 MHz signals that bounce aroundwithin the shipping container and through the substance to be detectedso that the stimulated emission response can be detected. Finally, longintegration times can be used to accumulate the stimulated emissions.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features of the subject invention will be betterunderstood in connection with the Detailed Description, in conjunctionwith the Drawings, of which:

FIG. 1 is a diagrammatic illustration of the detection of an explosivehidden on an individual as the individual walks through a balancedtransmission line coupled to an explosive/contraband detection unit thatutilizes nuclear quadrupole resonance in which, in one embodiment, RDXspectral lines are detected to ascertain the presence of an explosive;

FIG. 2 is a graph showing the spectral signatures of a number ofpotential explosive materials indicating for RDX and HMX, a spectralsignature in a 3-4 MHz range, with TNT indicating a spectral signaturein the sub 1 MHz range as well as ammonium nitrate and potassiumnitrate, with tetryl having a signature in the 3-4 MHz range and withurea nitrate having a spectral signature not only in the sub 1 MHz rangebut also in the 2-3 MHz range, noting that sodium nitrate has a veryclose spectral signature to one of the spectral lines of glycine;

FIG. 3 is a diagrammatic illustration a prior art pulsed coil nuclearquadrupole resonance system illustrating the use of high power pulsesand a high Q coil in which the system has a transmit-receive switch, thecycling of which depends on coil quench time;

FIG. 4 is a diagrammatic illustration of the subject system illustratinga stepped network analyzer functioning as a frequency source forgenerating a number of stepped frequencies which are amplified by a lowpower amplifier to less than 10 watts in one embodiment, with theamplifier being coupled to a balanced transmission line probe in whichthe transmission line is terminated in a load and in which a directionalcoupler is utilized to detect the stimulated emission from a materialunder test, unimpeded by the output power applied to the transmissionline;

FIG. 5 is a block diagram of the subject system in which transmissionsat various stepped frequencies are applied through a 24 bitdigital-to-analog converter to a circulator that functions as adirectional coupler, with the output of the circulator being convertedby a 24 bit A-D converter to correlate the returns with raw correlateddata from a library, the output of the hardware-implemented correlatorprovided to a microcontroller for detecting the existence of aparticular material present at the probe; Note that this system can beused to test several simultaneous frequencies simultaneously.

FIG. 6 is a diagrammatic illustration of an embodiment of the subjectinvention in which explosives detection includes the use of parallelfoil strips on opposing walls of a hallway that function as a balancedtransmission line probe for detecting target materials carried by aperson walking down the hallway;

FIG. 7 is a diagrammatic illustration of the utilization of a grid ofbalanced transmission lines for the location of a target materialcarried for instance by an individual who traverses the grid;

FIG. 8 is a diagrammatic illustration of the use of the subject systemas a nuclear quadrupole resonance component ratio detector for detectingthe ratio of molecular components in material proceeding down aproduction line to detect component ratios in a non-destructiveenvironment on the fly as the material passes between the balancedtransmission line probe elements;

FIG. 9 is a diagrammatic illustration of a shipboard containerinspection system utilizing the subject system in combination with amesh radio network to report incidents to a cargo control room;

FIG. 10 is a block diagram of parallel-connected frequency generatorscoupled to a terminated balanced transmission line;

FIG. 11 is a diagrammatic illustration of a wide area array grid ofterminated balanced transmission lines for detection of a sensedsubstance as well as location at a cross point;

FIG. 12 is a diagrammatic illustration of a balanced transmission linemethodology for feeding the transmission lines in parallel and fordetecting the position of a sensed substance either at or betweenbalanced transmission lines; and,

FIG. 13 is a diagrammatic illustration describing how Nuclear QuadrupoleResonance Techniques can be used to detect substances in a metalshipping container.

DETAILED DESCRIPTION

Prior to describing the subject shipping container-carried substancedetection system, the Nuclear Quadrupole Resonance Technique on whichthe subject invention is based is now discussed.

Referring now to FIG. 1, an individual 10 may be carrying on his or herperson some contraband or explosives 12 which may for instance may besecreted in his or her underwear, or could even be surgically implanted.One such explosive is RDX and it is the purpose of the subject inventionto be able to detect explosives in as little quantity as 75 grams whichis approximately about a fifth of a cup. Terrorists and the like areusing more and more sophisticated ways of secreting explosives and/orcontraband and a physical examination of the individual may not yieldthe presence of such explosives or contraband. Not only may theexplosives or contraband be surgically implanted in the individual, theymay be swallowed in bags and be held internally in the gut until suchtime as their “removal”.

Present systems for detecting such explosives or contraband such as backscatter X-rays are not effective to detect such secreted items and theuse of higher power radiation is counterindicated for safety reasons.

On the other hand, as shown in FIG. 1, an explosive or contrabanddetection system 14 utilizes nuclear quadrupole resonance in which sweptfrequencies are applied to a balanced and terminated transmission line16 embedded in a screening gate or housing 18 in which the elements ofthe balanced transmission line 20 and 21 as well as load 23 are embeddedin the gate. The balanced transmission line has no frequency to which itis tuned, such that the application of signals for instance between 100KHz and 10 MHz may be applied due to the non-tuned nature of the probewhich is comprised of elements 20, 21 and 23.

As will be seen, the power necessary to detect nuclear quadrupoleresonance is in general below 10 watts and often as little as 200milliwatts, due to this explosives/contraband detection system which,inter alia, utilizes a directional coupler in the form of a circulatorto cancel out the transmitted energy while receiving only the stimulatedemission from the molecules in the target sample.

As used herein, the target sample 12 includes molecules having aparticular recognizable spectrographic signature in which the spectrallines of the sample are recognizable when compared with the spectrallines generated through stimulated emission of all of the remainingmolecules that surround the target sample.

For instance, glycine which is common in the human body has spectrallines that are distinguishable for instance from RDX spectral lines,with glycine in essence forming a background spectral signature which isto be distinguished.

While the subject invention will be discussed in terms of explosives, itis understood that the material under test may be molecules of any typehaving a known spectral signature. This includes contraband such asnarcotics and other types of drugs such as heroin and cocaine which, dueto the subject system in one embodiment involving stepped and sweptfrequency transmission enables one to eliminate the Spectral signaturesof non-target materials while being able to single out the spectra oftarget materials.

Referring to FIG. 2, what is shown is a spectral chart for commonexplosive materials such as RDX, HMX, PETN, TNT, ammonium nitrate,potassium nitrate, tetral, urea nitrate and sodium nitrate, also ascompared with the spectra of glycine.

What will be seen is that all of these materials have spectra betweenabout 100 KHz and about 5 MHz, which spectra are detectable by thesubject system. For instance, if one detects spectra of RDX in the 3-4MHz band, this is clearly distinguishable from the glycine spectra whichlie below 1.5 MHz.

Likewise one can distinguish PETN from RDX as well as being able todistinguish HMX from RDX due to the offset of the spectra of HMX in the3-4 MHz band from the spectra of RDX.

Since this system detects stimulated emission from all of the moleculesin the sample between the balanced transmission lines, it is possiblethrough correlation processing to be able to provide a probability of amatch between the spectral lines of the target material as opposed tothe spectral lines due from molecules that are not target materials andwhich constitute background.

Referring now to FIG. 3, what will be seen in the prior art pulsed coilnuclear quadrupole resonance system is the utilization of a high Q coil20 which is driven from a frequency generator 22, the output of which isamplified by an amplifier 24 to the 1 kilowatt level. The signal fromthe amplifier is switched via a transmit/receive switch 26 and isapplied to the coil during a pulsed sequence, with switch 26 beingreturned to the receive position at which point the high Q coil 20 iscoupled to a low noise amplifier 26, to an analog-to-digital converter28 and thence to a computer 30 for measuring the spontaneous emissionresponse from material under test 32.

In short, since the system described in FIG. 3 measures the spontaneousemission of the material under test and since in order to generateenough spontaneous emission high power was deemed to be necessary, thesystem of FIG. 3 is clearly not usable around human beings for safetyreasons.

Moreover, in order to be able to eliminate the effect of the transmittedpower with respect to the relatively low power of the receive signal, itWas necessary to be able to quench high Q coil 20 so as to be able tosee the return from the material under test. The quench time, τ_(Q) isproblematic with respect to providing realtime measurements. It has beenfound that it is important to be able to provide circuitry to be able toquench high Q coil 20 in order to increase the pulse repetitionfrequency. However, the quench time when utilizing a high Q coil isproblematic as mentioned above.

Moreover, the utilization of a high. Q coil is problematic because italso collects background, which background can oftentimes obscure theresults.

On the other hand and referring now to FIG. 4, a balanced transmissionline probe 40 is coupled to a power amplifier 42 which amplifies afrequency generator 44 output, in one embodiment provided by a steppednetwork analyzer. The transmission line is terminated by a terminatingload 46.

When a material under test 48 is placed between the balancedtransmission line elements 50 and 52, it has been found that thestimulated emission from the material under test can be sensed utilizinga directional coupler 54 coupled to a low noise amplifier 56 which is inturn coupled back to the network analyzer 44 that detects a S21 the verylow level stimulated response of the material under test. It is notedthat network analyzer 44 is coupled to a computer 58 such that thereturned signal can be processed and an alarm 60 activated if thematerial under test has a spectral signature match to that of a targetmaterial.

While it is possible to generate only one frequency corresponding to onethe major spectral line of the target sample, it is useful to be able toscan frequencies for instance f₁-f_(n) in Order to detect the spectrallines of whatever materials might be between the elements of thebalanced transmission line. Because the balanced transmission line has aQ of zero, not only is it possible to couple a wide frequency range ofsignals to the transmission line, the Q of zero also means that there isvery little outside interference with respect to the signals that existinterior to the transmission line.

Moreover it has been found that while the flux densities vary at variouspositions between the transmission line elements, at least in the planeof the transmission line elements, locating a material under test aboveor below the plane of the transmission line elements does not materiallyaffect the readings.

Referring to FIG. 5, in one embodiment an memory card (such as a SXDX 62gigabyte card) having a 30 MB per second transfer rate may be utilizedto generate the 100 KHz to 10 MHz signals that are coupled to probe 64utilizing a 24 bit digital-to-analog converter 66 to which is applied aPN code 68 in one embodiment.

The utilization of a pseudo-random code is for defeating jamming, withthe pseudo-random code being similar to that utilized in GPS systems forthis purpose.

The input to the probe and the output from the probe are coupled to acirculator 70 which, as described above, completely eliminates theeffect of the transmitted signal on the received signal, thereby toeliminate the problems of having to quench a high Q coil.

The output of circulator 70 is applied to a 24 bit analog-to-digitalconverter 72, with the receive PN code being applied to a hardwareimplemented correlator 74 that correlates the received stimulatedemission information with raw correlator data 76 such that if there is ahigh correlation between the raw correlator data and the received data,microcontroller 78 may be used to drive memory card event log 80 andalso provide an operator interface 82 alarm condition indicator.

Note that in terms of the generation of stepped frequency signals, alibrary 84 may be utilized that carries the spectral signatures of manytypes of target molecules. This results in the ability to generate alarge variety of very narrow frequency signals which are applied toprobe 64.

It will be appreciated that the frequency stability of the signalgenerator in the form of a network analyzer such as shown in FIG. 4 iscritical due to the narrow nature of the spectral lines that aregenerated by the nuclear quadrupole resonance phenomena and therequirement of coherence.

Referring now to FIG. 6, in one embodiment, an explosive contrabanddetection system 90 may be coupled to a balanced transmission line probe92 which includes elements 94 and 96 embedded foil strips in hallwaywalls 98 and 100, with elements 94 and 96 terminated in a resistanceload 102. In this case an entire hallway may be monitored for thepresence of target molecules whether carried by a person or in someother conveyance as it transits down a hallway.

Referring to FIG. 7, it is possible to provide a grid of balancedtransmission lines here shown at 110 to include pairs of transmissionlines for instance vertical pairs 112 and 114 indicated by the plus andminus nomenclature for the particular transmission line. Likewise, acrossing or transverse transmission line structure may includetransmission lines 116 and 118. By monitoring the results on the varioustransmission lines one can localize the target molecule as illustratedat 120 as being at position x_(n) y_(m). This kind of grid, whether onthe floor or surrounding a building can track the presence of explosivesor contraband materials and therefore determine the track or path of theindividual or conveyance which is transporting these materials.

For this particular embodiment the detection of explosives in forinstance the north/south direction here illustrated at 122 is correlatedwith at explosive detection in east/west direction here illustrated at124 to provide location.

More particularly and referring now to FIGS. 11 and 12, in order to beable to geolocate a sensed substance, for instance carried byindividuals 202 and 204, a blanket array, wide area, detection grid 206stretches across an area 208 for instance in front of a building 210having an ingress 212.

When the subject system is utilized to protect an area from intrusionespecially by those carrying explosives, the wide area blanket arraydetection system includes balanced transmission line plus/minus pairs212 in spaced adjacently one to the other running horizontally asillustrated, whereas a crossed set of balanced transmission line pairs214 overlays the balanced transmission line pairs 212 so as to form theaforementioned grid.

In the horizontal direction and as mentioned before, a stepped networkanalyzer measuring board 220 includes a stepped network analyzer 222coupled to an amplifier 224 which provides a balanced output 226 and 228to a horizontally running, balanced transmission line comprised ofplus/minus lines 230 and 234. A directional coupler 236, coupled to alow noise amplifier 238 is connected as an input to the stepped networkanalyzer which measures S21 in a particular horizontal direction, hereshown by Y_(m).

For the vertical direction, a network analyzer board 240 includes astepped network analyzer 242 coupled to an amplifier 244 having abalanced output 246 and 248 coupled to balanced transmission lineelements 250 and 252. Here a directional coupler 254 is coupled to a lownoise amplifier 256, which provides a signal coupled back to steppednetwork analyzer 242 such that S21_(xn) is measured.

The above establishes a cross point array or grid with each of thebalanced transmission lines is terminated by a resistor 258.

Assuming that the stepped network analyzers are coupled to alocalization computer 260, its output provides the cross point locationon the grid.

As will be discussed, if a grid type of balanced transmission line arrayis not used, then the sensed substance can be located by detecting thephase between the transmitted and the received signal associated with abalanced transmission line, thereby to place the sensed substance at acalculated distance from the balanced line feedpoint.

In operation, the network analyzer board sends a swept CW signal down abalanced transmission line. If sensed substances such as explosives arepresent, a stimulated emission is picked up by the directional coupler.S21 is continuously measured and compared to an S21 sweep in memory,taken where no substances or objects are inside the transmission lines.The phase of the S21 output depends upon the distance of the detectedsubstance from the fed end of the transmission line. This providesinformation about the location of the sensed substances or explosives.

It is noted that the network analyzer board houses a power amplifier.

For each of the balanced transmission lines in the horizontaldirectional and each of the balanced transmission lines in the verticaldirection, there is a separate network analyzer board coupled to therespective balanced transmission lines. These boards simultaneouslytransmit the CW signals down the balanced transmission lines, with eachof the lines carrying CW signals of identical frequency and, in oneembodiment, zero phase difference between the signals.

Thus, each of the balanced transmission lines in the horizontal and thevertical direction are simultaneously fed with signals having identicalfrequencies. Moreover, the signals coupled to the balanced transmissionlines are in phase. As a result, if there is a sensed substance inbetween the lines of the balanced transmission line then there will bean S21 signal indicative of the substance.

Note, network analyzer board 220 is duplicated as illustrated at 220′whereas network analyzer 240 is duplicated as illustrated by networkanalyzer board 240′. Any significant difference between the S21measurement with the S21 values in memory provides an indication of asensed substance, in one embodiment, in a crossed point detectionsystem.

Referring to FIG. 12, it can be seen that a central processor 270controls analyzer boards 272, 274 and 276 coupled respectively tobalanced transmission lines 278-280, 282-284 and 286-288.

As mentioned with respect to FIG. 11, all transmission lines are fedin-phase. It is noted that the measured S21 information from eachanalyzer board is routed to a central processor.

If for instance a Perpetrator 2 is located between transmission lines282 and 284, Perpetrator 2 will be detected by analyzer board 2.

If however Perpetrator 1 is between the balanced transmission lines278-280 and 282-284, then Perpetrator 1 will be seen by both analyzerboard 1 and analyzer board 2.

The ratios of the signature reflection from analyzer board 1 andanalyzer board 2 is a function of exactly where Perpetrator 1 islocated.

It will be seen that using the ratios and the phases of the signaturereflections it is possible to track perpetrators as they progress alongthe floor Or across the array of balanced transmission lines.

It will also be noted that in one embodiment, the analyzer boards arecontrolled to provide a common sweep by central processor 270.

Referring now to FIG. 8, one of the important characteristics of thissystem is that the molecular component ratio can be detected on the flyin a production line environment to provide non-destructive testing.Here a nuclear quadrupole resonance component ratio detector 130 isutilized with a balanced transmission line probe 132 to, for instance,detect the molecular composition of a drug 134 in pill form as the pillspass through the balanced transmission line probe. It has been foundthat by sweeping the frequency of the signals to the balancedtransmission line probe one can detect not only the spectral lines ofthe various components in question, but also can detect the ratio of thetarget components.

Thus, rather than having to perform destructive tests in order toascertain the constituents of a product being manufactured, one cannon-destructively detect the component ratios utilizing the subjectnuclear quadrupole resonance system.

Shipping Container NQR Detection

Referring now to FIG. 9, another embodiment of this system is theability to track the contents of cargo containers that may either beplaced shipboard or on other modes of conveyance in which, asillustrated, a cargo container 140 may be provided with internalbalanced transmission lines 142 terminated as illustrated at 144 andcoupled, for instance, to an explosive detection system 146 of thesubject nuclear quadrupole resonance variety. If for instance thecontainers contain explosives or contraband, here illustrated at 148,whether these materials are initially placed in the container or laterclandestinely placed into a sealed container, their presence can bedetected as illustrated at 146 by an explosives detector. Through theuse of a mesh network 148, the detected results can be communicated fromexplosives detector 146 and a co-located transmitter 150 which is partof a self establishing mesh network 152 aboard a ship to the cargocontrol room. Mesh network 152 includes one or more repeaters 156 whichrelays the information from transmitter 150 to a receiver 158 in thecargo control room.

It is noted that when monitoring containers, due to the length of timeon board ship, the integration times available for the sensing of thestimulated emissions are dramatically increased. This long integrationtime can accommodate lower power detection. What this means is that anexceedingly robust system is available for detecting the relativelyminute simulated emissions, with integrating occurring over a longperiod of time, thanks to the fact that the containers are in transitfor substantial periods of time. While this embodiment of the subjectsystem has been described in terms of shipboard containers, any kind ofcontainer monitoring on conveyances is within the scope of the subjectinvention.

It is also possible for instance to utilize the subject system to detectcontraband or explosives in trucks that pass through a portal. This ispossible due to the relatively thick skin depths associated with metalcontainers that permit penetration of low frequency signals so that thetransmission line carried signals can penetrate well into thecontainers. Thus, the subject system may be utilized to detect not onlyperson-carried contraband and explosives, but also truck orvehicle-carried contraband or explosives, as for instance they proceedthrough a portal or checkpoint.

More particularly and referring now to FIG. 13, if it is desirable tocheck the contents of a shipping container such as a metal container 300carried by a truck 302, a nuclear quadrupole resonance detection systemof the type described above, here shown at 304, is connected to aterminated balance line array 306.

While the subject system will be described in terms of a truck, forinstance at a checkpoint in which the integration times may be as muchas 500 seconds, or in terms of a truck going 30 MPH across transmissionlines that extend 100 feet, giving a 2 to 3 second integration time, ithas been found that the near field from the balanced transmission linesextends upwardly without a great deal of attenuation in terms ofdistance to permit substance detection above the grid.

Thus, if truck 302 has a bed that extends 5 feet above the grid which ison a road having an implanted grid, then there is not much loss withrespect to distance.

If for instance the grid transmission lines are driven at 500 watts, anon-harmful amount of power given the fact that the grid is onlyradiating within the confines of the grid and not beyond the grid, thenit is possible to detect relatively large amounts of explosives orcontraband here shown at 308 confined within metal container 300.

As will be seen, measurements taken for steel containers of 1/16, ⅛ and¼ inch thickness, given 5 feet from the grid, results in the ability todetect substances within the metal shipping container. Thus if one forinstance loses 20 to 30 dB when traversing into a steel container, it isstill possible to detect the substances within the steel container.

The reason for this is, as noted above, is that at low frequencies skindepths are very deep, e.g. 10 mils. This means that as the skin depthgoes up so does the penetration. Thus, metal containers are relativelytransparent as 0.1-4 MHz.

As can be seen from the table below, the ability to detect sizableamounts of explosives or contraband within the metal container ispossible utilizing the subject NQR detection system.

TABLE I ESTIMATED POWER REQUIRED TO DETECT 500 KG OF EXPLOSIVES IN ACONTAINER 5 FEET ABOVE THE GRID STEEL THICK- FREO(KHZ) NESS 500 khz 700khz 1000 Khz .0625  1 SEC SWEEP .035 w .064 w 0.2 w 10 SEC INTEGRATION.011 w .02 w .063 w .125  1 SEC SWEEP 1.5 w 5 w 30 w 10 SEC INTEGRATION.47 w 1.6 w 9.5 w .1875  1 SEC SWEEP 50 w 400 w 5000 w 10 SECINTEGRATION 16 w 126 w 1581 w

Assuming as shown at 310 that one integrates the S21 results from thenetwork analyzers within the NQR detection system, then the robustnessof the substance detection revolves around the number of sweeps of asystem in the particular integration time.

The more sweeps that one can integrate, the better will be the results.Thus, if there is enough time to do 1,000 sweeps then the quality of theresults will be improved. Note, quality is directly proportional tointegration time. It is of course possible for stationary metalcontainers to integrate up to for instance 1,000,000 sweeps.

Such a situation would exist when metal shipping containers are aboard aship whose transit time may be measured in terms of weeks or months. Howone gets the information from each of the shipping containers outthrough to the cargo control room is a matter of utilizing meshnetworks.

Regardless, if one has a significant amount of integration time,substance detection 312 is indeed robust.

On the other hand, if one is trying to detect contraband or explosivesin a rolling vehicle which is proceeding down a road into which an array306 is embedded, then it depends on the velocity of the vehicle as tohow much integration time one has for a given length of the grid.

As can be seen, for a grid having a longitudinal dimension of 100 feet,and given a speed of 30 MPH or 44 feet per second, one has 2 to 3seconds of integration time.

As mentioned above, if the container is stationary, then it depends onhow long the container remains stationary. At a checkpoint for instancethere may be 500 seconds of integration time corresponding to the lengthof time that the vehicle is stopped at a checkpoint. The number ofsweeps at 500 seconds of integration time is 250. Moreover, if forinstance one is looking at a vehicle traveling 30 MPH, then one might beable to accomplish a few hundred sweeps.

Utilizing coherent integration techniques one can pick up another 20 dBin signal-to-noise ratio such that in all of the above cases, largeamounts of explosives or contraband can in fact be detected.

When metal container 300 contains substance 308 therein, assuming that a0.1-4 MHz signal 320 enters container 300 as indicated, it bounces backand forth within the container as illustrated by dotted lines 320′ suchthat there are multiple passes of the 0.1-4 MHz energy through thesubstance. The result of the multiple passes increases the amount ofstimulated emission from the substance such that a leaked out signal 322is detectable outside of the shipping container.

While the amount of leaked signal is attenuated by the passage throughthe metal walls of the container, it is a finding of the subjectinvention that the existing leaked signal is detectable and is no moreproblematical then the loss of 20 to 30 dB for long range stimulatedemission detection such as described in co-pending patent applicationentitled Long Distance Explosive Detection Using Nuclear QuadrupoleResonance and One or More Monopoles, corresponding to provisional PatentApplication Ser. No. 61/299,646.

It is noted that in one embodiment one can cover the whole frequencyrange between 0.1 and 4 MHz in 2 to 3 seconds, with this constituting asweep in one embodiment.

While it is possible to integrate the results from the directionalcouplers associated with the NQR detection system, it is also possibleto integrate the S21 ratios that are the result of the stimulatedemission. Moreover, it is possible to integrate the number of hits overtime for matching a particular signature with one of a library ofsignatures.

Thus, it is possible to integrate results from the directional couplersor the S21 ratios. It is also possible to integrate the output of thematched filters associated with particular explosives, there being alibrary of matched filters associated with the signatures, i.e. the setof resonances for each explosive, for instance as seen in FIG. 2.

Regardless of having large integration times, it is a finding of thesubject invention that relatively large amounts of substances to bedetected which are housed in a metal container can in fact be detectedby the subject NQR system, given an appropriate power level of signalcoupled to the terminated balanced transmission lines.

On the other hand, if one is utilizing radiated power from a monopoletuned to the particular sweeped frequency, then it is possible utilizingthis type of antenna structure to detect the presence of substanceswithin a vehicle that pass within for instance 50 or 60 feet of an arrayof monopoles.

In summary, it is a finding of the subject invention that it is indeedpossible to detect substances in metalized containers through the use ofthe subject NQR detection techniques that involve measuring thestimulated emission and matching signatures from the returned signalswith those in a library to derive a signal indicative of the presence ofa particular substance.

Referring now to FIG. 10, while the subject system has been described interms of stepped frequency production, it is possible to use aparallel-connected set of frequency generators 170, 172 and 174, theoutputs of which are summed at 176 and applied to a balancedtransmission line 178 having elements 180 and 182 through a circulator184. It is also possible to synthesize multi frequency signalsdigitally. The output of circulator 184 is applied to a network analyzeror receiver 186 that, inter alia, enables correlations between spectrallines found at the various frequencies to target molecule spectrallines, whereupon signals representative of the presence of the targetmolecule may be applied to an alarm 188.

Thus, whether or not stepped frequencies are utilized, or whether anumber of parallel-connected frequency sources are utilized, spectrallines of target and non-target molecules can be quickly scanned.

While the present invention has been described in connection with thepreferred embodiments of the various figures, it is to be understoodthat other similar embodiments may be used or modifications or additionsmay be made to the described embodiment for performing the same functionof the present invention without deviating therefrom. Therefore, thepresent invention should not be limited to any single embodiment, butrather construed in breadth and scope in accordance with the recitationof the appended claims.

1. A method for detecting substances including contraband and explosiveshoused in a metal container, comprising: irradiating the container withelectromagnetic energy below 4 MHz; and, measuring the stimulatedemissions from the substance that is the result of nuclear quadrupoleresonance.
 2. The method of claim 1, wherein the step of measuringnuclear quadrupole resonance from stimulated emissions includesmeasuring irradiation below 4 MHz.
 3. The method of claim 2, whereinmeasuring the stimulated emissions includes measuring the stimulatedemission spectra of the returns.
 4. The method of claim 3, wherein themeasuring step includes providing a network analyzer having thecapability of generating a signal below 4 MHz coupled to an antenna andhaving a directional coupler for separating the transmitted signal fromthe stimulated response signal, the directional coupler being coupled tothe network analyzer such that stimulated emission response is measuredby the S21 ratio.
 5. The method of claim 4, wherein the electromagneticenergy is frequency swept and wherein the S21 ratio response constitutesa stimulated emission signature, with the stimulated emission signaturebeing a result of the frequency sweep.
 6. The method of claim 5, andfurther including the step of initializing the measuring system byhaving the network analyzer transmit a 0.1-4 MHz signal into an areawhich is free of any substances to be detected, thus to establish abaseline.
 7. The method of claim 6, wherein the measuring step includesmaintaining a library of signals each related to a spectrum of adifferent substance, and further including the step of correlating thedetected signal with all the signals in the library such that a signalmatch indicates the presence of the indicated substance, whereassignatures that do not match are rejected.
 8. The method of claim 1,wherein the irradiated energy is provided by driving a terminatedbalanced transmission line.
 9. The method of claim 1, wherein theirradiated energy is providing by driving a monopole.
 10. The method ofclaim 9, wherein step of driving the monopole includes interposing atuner between signal source and the monopole and setting the tuner forthe instantaneous frequency of the signal source.
 11. The method ofclaim 10, wherein the signal from the signal source is frequency swept,whereby the tuner is correspondingly swept.
 12. The method of claim 1,wherein the metal container is a shipping container.
 13. The method ofclaim 1, wherein the shipping container is immobile over an extendedperiod of time and wherein the stimulated response is integrated over anextended period of time, the integrating detected step associatedintegarting the stimulated emission response.
 14. The method of claim 1,wherein the shipping container is aboard a vessel.
 15. The method ofclaim 13, wherein the detecting step includes providing a suitableantenna at or adjacent each shipping container.
 16. Apparatus for thedetection of explosives or contraband substances within a metal shippingcontainer, comprising: a frequency generator for generatingelectromagnetic energy below 4 MHz; an antenna coupled to said frequencygenerators for projecting energy below 4 MHz at said antenna; adirectional coupler coupled to said antenna; and, a stimulated emissiondetector coupled to said directional coupler.
 17. The apparatus of claim16, wherein said frequency generator is frequency swept.
 18. Theapparatus of claim 17, and further including a library of stimulatedemission spectra signature for specific substances and a correlator forcorrelating the signature associated with the output of said directionalcoupler and said library of signatures for indicating a match between asignature in the library and the signature associated with thestimulated emission, thereby to robustly detect a particular substancewhile rejecting the signatures associated with other substances.
 19. Theapparatus of claim 16, wherein said frequency generator includes anetwork analyzer and further including an amplifier coupled to saidnetwork analyzer for amplifying the signal from said network analyzer.20. The apparatus of claim 19, wherein the output of said amplifier issufficient to trigger a detectable stimulated response of substanceswithin said metalized container.
 21. The apparatus of claim 20, whereinthe power of said amplifier is proportional to the distance of saidantenna to said container.
 22. The apparatus of claim 16, wherein theamount of the substance to be detected in said metalized containerexceeds 500 pounds.
 23. The apparatus of claim 16, wherein saidcontainer is carried by a truck.
 24. The apparatus of claim 16, whereinsaid container is carried by a sailing vessel.
 25. The apparatus ofclaim 16, wherein said stimulated emission response is integrated.